1. Field of the Invention
The present invention relates to a nonwoven sheet consisting of polyethylene terephthalate (designated as "Polyester" hereinafter) filaments, constructed by entangling the filaments in a three-dimensional state, and having at least one smooth filmy surface layer, more specifically to a tenacious polyester filament nonwoven sheet having at least one smooth filmy surface layer, bulkiness, resistance to fuzzing by friction, and a high tear strength.
2. Description of the Related Art
Nonwoven sheets are used currently as printing substrates and packing materials. A nonwoven sheet (Japanese Examined Patent Publication (Kokoku) No. 4219520 ) constructed of extra fine polyolefin filaments is used widely because of its desirable smooth surface. Constructed of polyolefin filaments, this nonwoven sheet is inferior in printability and heat resistance. Furthermore, constructed of extrafine filaments, this nonwoven sheet has low tear strength. That is, the finer component filaments give a structure having a highly smooth surface, however, reduce the tear strength of the structure. To produce a heat resistant nonwoven sheet having a smoother surface and a higher tear strength, trials have been made to form a smooth surface by using a nonwoven sheet consisting of drawn polyester filaments. According to a known process for smoothing the surface of a nonwoven sheet, the surface filaments are heat-pressed for adhesion with a roll having a smooth surface.
In this process, the surface of the nonwoven sheet needs to be heat-pressed at a temperature near the melting point of the component filaments to smooth the surface. At such a critical temperature, the component filaments melt and are changed into a resinoid state, and hence the nonwoven sheet thus produced is embrittled. On the other hand, under a heat-pressing condition which will not resinify the component filament, the surface of the nonwoven sheet is merely flattened, and hence a satisfactorily smooth surface cannot be formed and the surface is liable to become fuzzy when subjected to friction, due to the weak bonding between the filaments.
Another method of producing a nonwoven fabric having a smooth filmy surface layer is known from Japanese Examined Patent Publication (Kokoku) No. 4841115. In this method, a nonwoven fabric constructed of polyester filaments with a second order transition point below room temperature is heat-pressed to make the surface thereof smooth. In this nonwoven fabric, since the second order transition point of the filaments is below room temperature, the resistance to heat of the nonwoven fabric is low so that the nonwoven fabric is not suitable for practical use.
According to still another process, the surface of a nonwoven sheet is coated with a smooth resin layer to form a smooth surface. This process, in general, reduces the tear strength of the nonwoven sheet, though this is dependent on the type and quantity of the resin used.
In view of the problems in the prior art, the inventors of the present invention intended to form a smooth surface by using heat-resistant filaments which can readily be deformed by heat-pressing, such as undrawn polyester filaments of a low softening point. Although a smooth surface could be formed, simple heat-pressing treatment of the web consisting of such filaments produced a stiff and lean nonwoven sheet of an extremely low tear strength, because of the general flattening of the filaments of a low softening point.